1. Field of the Invention
The present invention relates to a plasma generating apparatus and method and, more particularly, to an apparatus and method for generating a plasma used for the purpose of conducting a process, e.g., etching or film formation in, e.g., semiconductor device manufacturing processes.
2. Description of the Related Art
In recent years, a large-scale integrated circuit (LSI) obtained by integrating a large number of elemental devices, e.g., transistors and resistors, on one chip is often employed in the important portion of a computer or communication equipment. Hence, the performance of the entire equipment is closely related to the performance of the LSI. An improvement in performance of a single LSI can be realized by increasing the integration degree, i.e., by micropatterning the elemental devices.
An example of the technique effective for micropatterning the elemental devices includes plasma-assisted techniques, e.g., reactive ion etching and plasma CVD (Chemical Vapor Deposition) For example, with reactive ion etching, ions in the plasma can be vertically radiated on the etching target substrate. Thus, etching can be imparted with anisotropy, enabling formation of a micropattern.
Of LSIs, however, semiconductor devices expected to have a further increase integration degree in the future, such as a 256-Mbit or 1 Gbit DRAM of a next-generation, are coming to require a design rule of quarter micron (0.25 .mu.m) or less. Thus, in formation of a gate electrode constituting a transistor or in trenching for forming a device isolation region or a memory capacitor, etching capable of realizing microprocessing and a high aspect ratio is required. Conventional reactive ion etching is unlikely to satisfy these requirements.
FIG. 15 is a schematic diagram briefly showing the arrangement of a conventional inductive coupling plasma etching apparatus (Jpn. Pat. Appln. KOKAI Publication No. 5-206072).
As shown in FIG. 15, a susceptor 207 for placing a target substrate 206 thereon is provided in a process chamber 201. An RF (radio frequency) bias power supply 208 applies an RF bias to the susceptor 207. A quartz cylinder 202 for generating a plasma therein is provided on the process chamber 201. A coil 203 is wound on the outer surface of the quartz cylinder 202.
The process chamber 201 and the quartz cylinder 202 are integrally formed to constitute an etching room.
The coil 203 is connected to a first variable capacitor 205a and a second variable capacitor 205b to a parallel resonance circuit. One terminal of each of the first and second variable capacitors 205a and 205b is connected to an RF power supply 204. The RF power supply 204 supplies an RF power to the parallel resonance circuit.
The process chamber 201 is connected to a turbo-molecular pump 212 through a variable conductance valve 211. The process chamber 201 is provided with a pressure gauge 209. The opening degree of the variable conductance valve 211 is adjusted by using the pressure gauge 209, so that the pressure in the etching room can be set to a desired level.
A gas inlet pipe interposed with a massflow meter 210 is connected to the upper wall of the quartz cylinder 202. With the use of the massflow meter 210, a reactive gas can be introduced into the etching room at a desired flow rate.
An etching method using the plasma etching apparatus having the above arrangement will be described.
By using the massflow meter 210, a fluorocarbon-based process gas (e.g., CF.sub.4, C.sub.2 F.sub.6, C.sub.3 F.sub.8, or C.sub.4 F.sub.8) is introduced into the etching room at a predetermined flow rate.
By using the pressure gauge 209, the interior of the etching room is held at a predetermined pressure of about several Pa. In this state, the RF power supply 204 supplies RF power to the coil 203 to generate a plasma in the etching room.
By using the RF bias power supply 208, an RF voltage is applied to the susceptor 207. Thus, ions in the plasma are caused to be vertically incident on the surface of the etching target substrate 206 (silicon oxide film) to anisotropically etch it.
However, the conventional plasma etching apparatus of this type has problems as follows.
For example, when a small contact hole (a hole for connecting a wiring layer on the silicon oxide film with the silicon substrate under the silicon oxide film) is formed in the silicon oxide film, etching rate, etching selectivity (ratio of the etching rate of the silicon oxide film with respect to the etching rate of the silicon substrate) and the planar uniformity of etching anisotropy sometimes exceed the allowable ranges for manufacturing a semiconductor device. In particular, decreases in etching rate and etching selectivity become conspicuous at the central portion of the target substrate. If a stronger RF power is applied to increase electron density thereby increasing ion current incident onto the target substrate, the etching rate tends to be decreased and the planar uniformity of the etching selectivity tends to be worsened.
Generally, where a stronger RF power is applied, the etching rate and etching selectivity of a silicon oxide film are decreased. Furthermore, sometimes the etching rate may be changed depending on the hole diameter of the contact hole and, when the contact hole has a hole diameter on the order of sub-half micron, etching may be stopped midway in a silicon oxide film.
These phenomena may be caused by the following mechanism. That is, as the RF power is increased, the electron density is increased, and dissociation of the process gas is promoted. Then, ions of fluorocarbon of a low fluorine number are incident on the target substrate 206 and attach to the surface of an opening hole. This decreases the etching rate of the silicon oxide film, thereby decreasing the etching selectivity.
A portion of the quartz cylinder 202 close to the coil 203 during use is mainly eroded (erosion via SiO.sub.2). Chipped quartz is discharged into the plasma in the form of Si, SiOx, O, and the like. Accordingly, when plasma electric discharge is continued for a long period of time, e.g., for 100 hours or more by using CF.sub.4, the quartz cylinder 202 may cause fracture. Application of a strong RF power described above also poses the problem of promoting fracture of the quartz cylinder 202.
Fracture of the quartz cylinder 202 is caused because the closer to the coil 203, the more intensive electric field is formed. More specifically, the electric field near the sheath (sheath electric field) between the plasma and the quartz cylinder 202 increases, so that the high-energy ions in the plasma are adsorbed in the surface of the inner wall of the quartz cylinder 202, thereby causing erosion. When the wall thickness of the quartz cylinder 202 is decreased, the sheath portion further enters the intensive electric field region. Then, erosion progresses fast, and fracture of the quartz cylinder 202 progresses in an accelerated manner.